Since non-metallic inclusions in steels cause the occurrence of macro-streak-flaws or crackings which deteriorate the properties of steels, various studies have been made on a method of decreasing them and rendering them harmless by control of shapes. The non-metallic inclusions are mainly consist of oxides and sulfides such as Al2O3 and MnS. Therefore, enhanced cleaning and refining such as vacuum treatment of molten steels for oxides, and intensive desulfurization etc. for sulfides, have been used until this time to greatly decrease the amount of non-metallic inclusions. Further, it has been intended to render them harmless by controlling the shape of the remaining inclusions by Ca treatment, and the deterioration of the product properties, caused by non-metallic inclusions, has now been drastically decreased.
However, as the required strength has been increased and the working circumstances have become more severe, steels have become more sensitive to the effects of the non-metallic inclusions and it is now necessary to render the non-metallic inclusions further harmless in order to improve the properties of steels.
For example, in the case of steel pipes for the oil country tubular goods which are used in oil and/or natural gas wells, under the situation for energy demand and supply or the state of the existence of resources, the well-depth has been increased and the excavation under strongly acidic circumstance containing more hydrogen sulfide has been necessary. Therefore, the steel pipes having the higher strength and excellent resistance to sulfide stress cracking (SSC) are required.
Generally, as the strength of steels increases, the SSC resistance thereof is lowered. In order to improve the SSC resistance, countermeasures should be adopted for metal structures such as (1) refining a crystal grain structure, (2) increasing the area ratio of martensite phase in the microstructure, (3) increasing the tempering temperature, and (4) increasing the content of the alloying elements which have an effect of suppressing corrosion. However, even when such countermeasures are adopted, for example, in a case where harmful non-metallic inclusions are present, cracking tend to occur as the strength is increased.
Accordingly, in order to improve the SSC resistance in increased strength steels, an amount and a shape of non-metallic inclusions have to be controlled together with the improvement for metal structures.
The Patent Document 1 discloses the invention of a high strength steel pipe, having a yield stress of 758 MPa or more (110 ksi or more), in which the number of TiN inclusions with the diameter of 5 μm or more, is 10 or less per 1 mm2 in the cross sectional area. It describes that precipitation of the TiN has to be controlled in the steel pipe, having the yield stress of 758 MPa or more, since the TiN derived from Ti, which is added for improving the SSC resistance, is precipitated in a coarse form in the solidification process of the steel. This results in pitting corrosion in the portion on the steel surface where the TiN inclusions are exposed and it constitutes a starting point of SSC.
It is considered that, in a case where the grain size of the TiN is 5 μm or less or the density of occurrence of the TiN is small, the TiN does not form the starting point of corrosion. It is assumed that while the TiN is insoluble to acids, it functions as a cathode site in corrosive circumstances, since it is electrically conductive, to dissolve the matrix at the periphery to form the pitting corrosion, as well as to increase the concentration of occluded hydrogen in the vicinity and generate the SSC due to stress concentration at the bottom of pits. In view of the above, in order to make the grain size of the TiN inclusions 5 μm or less and the number thereof is 10 or less per 1 mm2, it is defined in the Patent Document 1 that the N content is limited to 0.005% or less, the Ti content is limited to 0.005 to 0.03% and the value for the product of (N %)×(Ti %) is limited to 0.0008 or less in the steel.
In addition, it has been well known that the addition of a trace amount of Ca or the application of a Ca treatment for molten steel has an effect of rendering the shape of inclusions harmless in steels with a decreased amount of O (oxygen) or a decreased amount of S; for example, by suppressing the formation of clusters of oxides such as Al2O3 or granulating MnS inclusions which tend to be extended. The Patent Document 2 discloses the invention of a low alloy steel, excellent in SSC resistance which forms fine Al—Ca inclusions by utilizing the effect of Ca and precipitating Ti—Nb—Zr carbonitrides around the inclusions as a nucleus, thereby controlling the grain size of the composite inclusions to 7 μm or less in the major diameter and dispersing them by 10 or more per 0.1 mm2.
The steel disclosed in the Patent Document 2 is produced by applying the Ca treatment to an Al deoxidized molten steel containing 0.2 to 0.55% of C, with an addition of a smaller amount of Ti, Nb and Zr, etc., and containing 0.0005 to 0.01% of S, 0.0010 to 0.01% of O, and 0.015% or less of N and controlling the cooling rate to 500 degrees C./min or less from 1500 degrees C. to 1000 degrees C. in the casting of the steel pieces.
Patent Document 1: Japanese Patent Laid-Open No. 2001-131698
Patent Document 2: Japanese Patent Laid-Open No. 2004-2978